(1) Filed of the Invention
The present invention relates to a method of draw-forming a metal sheet having an organic film. More specifically, the invention relates to a method of stably producing drawn cans and, particularly, drawn/redrawn cans without developing pinching, enamel hair or film hair at the time of draw-forming a metal sheet having an organic film, and without causing the drawn containers to be damaged or the tools to be worn out, but maintaining excellent produceability and formability.
(2) Description of the Prior Art
Seamless can barrels with bottom have long been produced in the field of producing cans by subjecting a metal sheet and, particularly, a metal sheet having an organic film to drawing or to drawing/redrawing.
The drawn cans or the drawn/redrawn cans have been produced by holding a metal sheet having an organic film using an annular holder member (often called blank holder) and a drawing die, moving the drawing die and a drawing punch relative to each other and in mesh with each other, the drawing punch being positioned so as to move in concentric relationship with the holder member and the redrawing die within the holder member, to thereby; draw-form the metal sheet with an organic film into a cup shape.
In the drawing/redrawing step, furthermore, a cup drawn from the metal sheet having an organic film through a preceding stage is held by an annular holder member that is inserted in the cup and by a redrawing die, and a redrawing punch and the redrawing die are moved in mesh with each other and relative to each other, the drawing punch being so provided as to move in concentric with the holder member and the redrawing die within the holder member, in order to obtain a deep-draw-formed cup having a diameter smaller than the cup drawn in the preceding stage.
In the step of redrawing, furthermore, a method has already been put into practice in which the radius of curvature of the redrawing die is sufficiently decreased to sufficiently increase the back tension that results from a combination of the holder member and the redrawing die, in order to decrease the thickness of the side wall portion of a final cup by bend-elongation (Japanese Laid-Open Patent Publication No. 258822/1989).
In draw-forming the metal sheet, in general, it is considered necessary to so form the cup that a flange is left whenever the draw-forming is carried out under the condition where a predetermined blank holding force is acting from the standpoint of preventing the formed cup from being wrinkled and producing cans having excellent shape and dimensional precision.
Under draw-forming conditions in which flanges remain in the formed cup, however, a limitation is imposed on the length of stroke of the punch as well as on the number of strokes of the punch per unit of time, i.e., on the speed of formation. It has therefore been desired not to have flanges remain, but rather to pull the flanges between the die, in order to enhance the rate of production.
In conventional draw formation or draw-redraw formation, the cup has been drawn with a predetermined blank holding force acting between the annular holder member and the drawing dies, i.e., to pull the flanges between the punch and the dies so that the flanges do not remain after drawing.
When the metal sheet having an organic film is drawn into a flanges cup, however there takes place a so-called selvage worn-out phenomenon (pinching) at the upper end of the cup or a phenomenon (film hair or enamel hair) in which the coating of the form of a film or enamel becomes like a waste of threads to cause damage compared to when the flanges are allowed to remain. In extreme cases, furthermore, the can barrel is broken or the organic film is broken during the drawing. Moreover, the tools that are used are subject to be worn out and damaged, and lose service life to a considerable degree.
When a cup drawn in the preceding stage is to be redrawn into a cup having a smaller diameter, in particular, the flanges remaining in the cup that is drawn in the preceding stage must be pulled into the plane of the holder member between the holder member and the redrawing die. Otherwise, the above-mentioned tendency appears conspicuously.